Black Caviar appeared exhausted after her win at Royal Ascot on Saturday but a veterinary examination gave her the all-clear. Photograph: Alastair Grant/AP

"That will give doubting Thomases some material," Peter Moody said, once he had had time to reflect on Black Caviar's win in the Diamond Jubilee Stakes here on Saturday, her first race outside Australia and the narrowest victory of her career. In scrambling home by a head, the mare showed admirable resilience but opened the door to questions about the true extent of her ability.

The pride of Australia remains unbeaten but, even if her jockey, Luke Nolen, had continued to push her out to the winning post, she would not have won with anything like the ease of her 21 successes at home. By the rider's own estimate, she would probably have finished about a length clear.

"I'm slightly disappointed for your public that they haven't seen how great this filly is," Moody said. "I think I saw the finest performance I've ever seen on a racecourse on Tuesday in the winner, Frankel. Maybe if I'd had this mare here last year, I would probably have said the same thing. Now, we're nearer the end rather than the start."

Though she appeared exhausted in the winner's enclosure, a veterinary inspection discovered nothing wrong with Black Caviar that might have caused her to underperform. But those who have claimed her as the fastest horse in the world will not be able to accept this as a fair measure of her worth. The winner of the next race, a lowly handicap, completed the same course in a faster time, carrying half a stone more than the supposed supermare.

Supporters of Frankel can have the last laugh, not just because their hero won by 11 lengths but because his top speed, as measured by Turftrax, was greater. He hit 42.53mph in a longer race, whereas Black Caviar's best was 41.5mph from the two-furlong marker to the one. The ground would not have been exactly the same for both races, though it was officially good by the time of Saturday's race, whereas Frankel ran on good to soft.

An immediate consequence of Saturday's outcome is that Black Caviar will not stay in Britain for next month's July Cup at Newmarket, which had seemed a long shot in any case. She will enter quarantine and return to Australia in a matter of weeks, with the possibility of more races this year if Moody is convinced of her wellbeing.

Retirement cannot be far away, now that she has come so close to losing her prized unbeaten record. "I think we'd like to breed from her," said Pam Hawkes, one of her owners, "and race the babies to create a Black Caviar dynasty." There is, she added, "no way" they could contemplate a sale. "We've been offered a lot of money for her but it's not about money."

The bloodstock expert Ed Prosser suggested Black Caviar might fetch £10m if sold at auction as a broodmare prospect, though he said it could be no more than a tentative estimate because of her status as "a trophy mare. It's hard to put a value, because if they go to sale, it's what two people decide she's worth."